1. Field of the Invention
The present invention relates to a tool for forming a hem flange.
2. Background Art
Vehicle body panels such as deck lids, hoods, doors and the like frequently include inner and outer panels that are secured together by means of a hem that extends about their periphery. Such body panels have traditionally been manufactured from steel sheet metal panels. Steel is very ductile and is easily formed in a hem forming operation. Increasingly, automotive manufacturers are turning to aluminum to obtain weight savings for vehicle body panels. Aluminum alloys offer a high strength/low weight alternative to steel.
Aluminum does not, however, have the same degree of ductility and resistance to work hardening offered by steel. Forming a hem on a sheet metal body panel made of aluminum is more difficult than forming the same hem with steel due to aluminum's reduced ductility in comparison to steel. One proposed solution to this problem was to form a larger radius hem when making body panels of aluminum sheet metal. Larger radius hems result in lower fit and finish ratings because larger radius hems cause gaps between door closure panels and their openings appear larger. Further, the low ductility of aluminum may cause tears or splits starting from the outer surface of a hem. Tears and splits result in high part rejection rates and unacceptable scrap rates.
Substantial work hardening may occur during the hem flange formation process. The hem flange formation process is the initial step in forming a hem wherein a peripheral portion of a blank or drawn part is bent to about 90°. Forming a 90° bend in an aluminum sheet around a relatively tight radius causes substantial amounts of deformation. This amount of strain results in splits and even tears as the hem flange is further formed in pre-hem and final hem forming steps.
These and other problems are addressed by applicants' invention as summarized below.